This invention relates to telephone extension systems providing a portable or mobile extension telephone which communicates over AC power wires. More particularly, the present invention provides apparatus for communicating over AC power wires between an extension telephone and a conventional telephone line.
Heretofore, it has been propsed to provide an extension telephone system communicating with a conventional on line telephone through transmission means other than an extending telephone line. Various wired and wireless telephone extension systems have been proposed. The wireless systems consist of transmitting and receiving radios at the on line telephone (which connects directly to the telephone line) and at the extension telephone (which is portable). These systems encounter unique problems relating to the power and frequency of radiated signals as well as other problems of signal loss between the on line and extension phones due to physical shielding between them. Furthermore, since all signals between the on line and extension telephones are transmitted, they can be intercepted freely by anyone within range.
The wired telephone extension systems that have been proposed make use of the AC power lines for communicating between the on line and the extension telephones. In these wired extension telephone systems making use of the AC power lines, the subscriber's on line telephone which is connected directly to the telephone line, is specially equipped with an acoustical speaker that sends audio signals to the on line telephone mouthpiece. These acoustical devices are contained in an auxiliary unit that goes alongside the on line telephone and which holds the telephone handset. A lever extends from this auxiliary unit to the cradle of the on line telephone where the handset usually rests, and actuates the operating button on that cradle which is usually actuated when the handset is manually lifted from or placed on the cradle. These acoustical devices communicate electrically with the mouthpiece and earpiece on the handset of the extension telephone, through the available AC power wires. This prior system uses carrier frequencies which are modulated by the acoustical signals. When this system is in use, an incoming telephone call can be answered at the extension telephone as well as at the on line telephone. However, the incoming call can not be answered at both the on line and the extension telephones, because when the on line handset is removed from the auxiliary unit, the acoustical couplings are broken and so the extension telephone is incapable of either receiving or transmitting to the on line telephone. Furthermore, the acoustical microphone in the auxiliary unit will pick up surrounding noise as well as the incoming voice signals that appear at the earpiece of the on line telephone handset and this noise will be transmitted along with the incoming voice signals over the AC power line to the extension telephone. Another shortcoming of this prior system is that calls cannot be placed by the extension telephone. There is no way provided for transmitting dial signals or dial tones from the extension phone to the subscriber's telephone line.